Pukete wastewater treatment pilot trials
In this issue we introduce the concept of constructed wetlands and the trials underway at the Pukete Wastewater Treatment Plant to test the performance of a number of constructed wetland modules at treating wastewater and to look at their resilience to shock loading.
When asked for advice on almost any subject, as scientists and engineers, we try to base our answers on the results of scientific research. So when we embarked on this project to advise marae and communities on appropriate wastewater treatment strategies, we considered the types of challenges typically faced - where there are intermittent peaks in wastewater flows - and what solutions may be available that could deliver good wastewater treatment.
We also focussed on options that are simple to construct, operate and maintain, and are cost effective. Based on our experience, and with these objectives in mind, we identified constructed wetlands and passive woodchip filters or bioreactors as potential components of onsite wastewater treatment systems. Here we provide a short summary of what we have been researching and the kinds of results we have been seeing so far.
Firstly, what is a wetland?
Natural wetlands are areas of land which are wet enough, often enough that the plants (and animals) in them have adapted to survive and thrive despite being in permanent or semi-permanent water, e.g., plants such as raupo, bulrushes, reeds etc. Wetlands are normally found at the interface between dry land and permanently flowing water like springs, streams, lakes, and estuaries. Water flowing from the land often passes through these wetland areas where it is cleansed of many of its contaminants –wetlands are sometimes referred to as the ‘kidneys’ of the landscape.
A constructed wetland is an engineered wetland, designed to serve some water treatment or management purpose. The simplest constructed wetlands are most similar to shallow swamps. They slow water flows and allow them to spread out, dropping any sediments they may be carrying, and removing nutrients by plant uptake and bacterial removal. People often think of plant uptake as the major way that nutrients are removed because the plants are highly visible, however after 6-12 months, bacteria generally become the major removal mechanism for nutrients such as nitrogen.
Nitrogen comes in a number of forms e.g. organic nitrogen (such as nitrogen in the food we eat), various dissolvable forms such as ammonium and nitrate (which are plant nutrients), and some gaseous forms such as nitrous oxide and N2. N2 gas makes up around 80% of the air we breathe, and is inert (stable and non-reactive). In a wetland we want to convert the organic and dissolvable forms of nitrogen into N2 gas. This normally involves first converting ammonium into nitrate (NH4 → NO3, or nitrification, basically adding oxygen), and then converting it to N2 gas (via denitrification, or removing the oxygen again under low oxygen conditions).
This is really the key to our trials – giving the wastewater aerobic (high oxygen) conditions, and then anaerobic (low oxygen) conditions to provide the right conditions to remove nitrogen from the water.
So now onto our trials. These were run at the Pukete Wastewater Treatment Plant which treats wastewater from Hamilton City. If you want to run a trial on wastewater, you need a steady supply, and they certainly have that. Our systems started with the simplest types of constructed wetlands, and then moved to more sophisticated set-ups.
The Basic Wetland System
The most basic system consisted of a gravel bed planted with wetland plants. Wastewater was added into one end and ran out the other. The water level was kept just below the surface. This is known as a horizontal flow gravel bed wetland.
With this system there is no ‘active’ addition of aeration (for nitrification), nor active way of creating low oxygen conditions (for denitrification). Even so, the system was pretty good, removing about half the nitrogen and more than 95% of solids and BOD (biochemical oxygen demand, a key measure of wastewater strength).
Advanced Wetland Systems
If you need to remove greater amounts of nitrogen, a more advanced system is required. By intermittently spraying incoming water over the surface of a sand or gravel bed wetland, and then allowing it to passively drain between applications, the wastewater is spread as a thin film over the gravel surface, allowing exposure to lots of oxygen. These types of wetlands are called Vertical Flow Wetlands (or VFW). Ammonium nitrogen (NH4) is readily nitrified to nitrate (converted into NO3) as they pass through the media - this step is typically called nitrification (Figure 2).
The next step following nitrification is denitrification where nitrate is converted to nitrogen gas (NO3 → N2 gas). This step requires extra energy, which the bacteria get from organic carbon. However, the VFW generally filters most of this out, so we have to provide an additional source of it, or catch it before we add the water to the VFW (and use it later). The easiest way is to provide an additional organic carbon source such as wood chips. We put these in a tank and run the nitrified wastewater through it. Because it is permanently filled with water less oxygen gets in and forces the bacteria to scavenge for oxygen off the nitrate (NO3).
We call these woodchip filters ‘Denitrifying Bioreactors’. We have a couple of different types we are trialling, but, in short, they now remove between 60% and 75% of the incoming nitrogen and also reduce faecal microbes.
We have also been testing these systems to see how well they cope with big jumps in the amount of waste they have to cope with (not uncommon for marae, camping grounds and beachside communities). We’ll give you an update on this in our next newsletter.